1. Field of the Invention
The invention relates generally to the field of drilling wellbores through the earth. More particularly, the invention relates to apparatus and methods for determining the dynamic mode of motion of a drill string used to turn a drill bit.
2. Background Art
Drilling wellbores through the earth includes “rotary” drilling, in which a drilling rig or similar lifting device suspends a drill string which turns a drill bit located at one end of the drill string. Equipment on the rig and/or an hydraulically operated motor disposed in the drill string rotate the bit. The rig includes lifting equipment which suspends the drill string so as to place a selected axial force (weight on bit—“WOB”) on the drill bit as the bit is rotated. The combined axial force and bit rotation causes the bit to gouge, scrape and/or crush the rocks, thereby drilling a wellbore through the rocks. Typically a drilling rig includes liquid pumps for forcing a fluid called “drilling mud” through the interior of the drill string. The drilling mud is ultimately discharged through nozzles or water courses in the bit. The mud lifts drill cuttings from the wellbore and carries them to the earth's surface for disposition. Other types of drilling rigs may use compressed air as the fluid for lifting cuttings.
The forces acting on a typical drill string during drilling are very large. The amount of torque necessary to rotate the drill bit may range to several thousand foot pounds. The axial force may range into several tens of thousands of pounds. The length of the drill string, moreover, may be twenty thousand feet or more. Because the typical drill string is composed of threaded pipe segments having diameter on the order of only a few inches, the combination of length of the drill string and the magnitude of the axial and torsional forces acting on the drill string can cause certain movement modes of the drill string within the wellbore which can be quite destructive. For example, a well known form of destructive drill string movement is known as “whirl”, in which the bit and/or the drill string rotate precessionally about an axis displaced from the center of the wellbore, either in the same direction or in a direction opposite to the rotation of the drill string and drill bit. Another destructive mode is called “bit bounce” in which the entire drill string vibrates axially (up and down). “Lateral” vibrations and “torque shocks” can also be detrimental to drill string wear and drilling performance. Still other movement modes include “wind up” and torsional release of the bottom of the drill string when the bit or other drill string components momentarily stop rotation and then release. Any or all of these destructive modes of motion, if allowed to continue during drilling, both decrease drilling performance and increase the risk that some component of the drill string will fail.
The foregoing examples are not intended to be an exhaustive representation of the destructive movement modes a drill string may undergo, but are only provided as examples to explain the nature of the present invention. It is known in the art to measure axial and lateral acceleration or related parameters, as well as axial force and rotational torque related parameters, at the earth's surface to try to determine the existence of a destructive mode in the drill string. A limitation to using surface measurements to determine destructive drill string modes is that the drill string is an imperfect communication channel for axial, lateral and/or torsional accelerations which are imparted to the drill string at or near the bottom of the wellbore. In particular, the drill string itself can absorb considerable torsion and change in length over its extended length. Moreover, much of the drill string may be in contact with the wall of the wellbore during drilling, whereby friction between the wellbore wall and the drill string attenuates some of the accelerations applied to the drill string near the bottom of the wellbore.
It is also known in the art to measure acceleration, rotation speed, pressure, weight and/or torque applied to various components of the drill string at a position located near the drill bit. Devices which make such measurements typically form part of a so-called “measurement-while-drilling” (MWD) system, which may include additional sensing devices for measuring direction of the wellbore with respect to a geographic reference and sensors for measuring properties of the earth formations penetrated by the wellbore. A limitation to using MWD systems known in the art for determining destructive operating modes in a drill string is that the data communication rate of MWD systems is generally limited to a few bits per second. The low communication rate results from the type of telemetry used, namely, low frequency electromagnetic waves, or more commonly, drilling mud flow or pressure modulation. The low communication rate requires that very selected information measured by various sensors on the MWD system be communicated to the earth's surface by the telemetry (known in the art as “in real time”). Destructive modes, however, may include accelerations having frequencies of several Hertz or more. Typically, measurements of acceleration, rotation speed, pressure, weight and/or torque are sampled at a relatively high rate, but only average amplitude, average amplitude variation or peak values are transmitted to the earth's surface without regard to whether a peak, average or average variation value corresponds to any particular drill string failure mode. As a result, MWD systems known in the art do not necessarily make the best use of the mode-related measurements made by the MWD system sensors.
It is desirable to have a method and system for identifying drill string movement modes that can communicate the identified mode to the earth's surface for analysis so as to facilitate the appropriate remedial action for each specific movement mode and reduce the chance of drill string failure.